Conventionally, vehicle bodies such as a motor vehicle have been known in which plural laminated metal sheets are joined together through spot welding at plural locations (refer to Patent Literature 1, for example). However, in forming the vehicle body through spot welding, a larger number of welding spots are required as the size of a member to be welded increases, making it difficult to improve the work efficiency.
On the other hand, as one of resistance welding methods, the seam welding is known, and this seam welding is used for fabrication of a truck's fuel tank, for example (refer to Patent Literature 2, for example). The seam welding is executed by holding plural laminated metal sheets by roller electrodes made up of a pair of circular disc members on side surfaces of the circular disc members, causing the roller electrodes to roll on the metal sheets while causing the roller electrodes to press against the metal sheets with a predetermined pressing force and energizing the roller electrodes. The roller electrodes are, for example used by being attached to an arm of a welding robot. In the seam welding, the roller electrodes are controlled so as to roll on a planned welding portion which is preset. Consequently, according to Patent Literature 2, nuggets are formed continuously between both the roller electrodes by using the seam welding, whereby it is said that airtightness and watertightness are imparted to the fuel tank.
Additionally, in the seam welding, in forming a continuous closed state, that is, an endless welded joint portion, a wide welding area can be ensured, and hence, the seam welding is said to be useful in improving the work efficiency.